How to edit a shapekeys lerp?
Shape keys, Relative or Absolute, may only be edited at 1. That is their fixed point of reference. In the case of Relative shape keys, what is stored is the difference between that fixed point of reference and whatever is in the 'Relative To' field of the key.

Relative interpolations are linear. Setting a value outside 0-1 is calling for a linear extrapolation, beyond 0-1, of the imaginary straight lines in object space down which vertices travel between value 0 and value 1.



Using 'New Shape from Mix' in the menu under the little down chevron on the right of the keys list, you can take your -1 extrapolation of the 'Tall' shape key and make it into a key in its own right; say.. 'Short'.

You can now edit the Short key.

How you deal with the keys after that is up to you.. if you want to return to just 2 keys, you could, for instance, delete the Basis, and have a single relative key from 0=Tall to 1=Short, but beware, the whole journey (the new straight line) will not pass through what was 'Basis'before.

You may go for Absolute keys, perhaps re-ordering them. In that case 'Tall', 'Basis'and 'Short' will be put in a single continuum, and you will have to alter only one 'Evaluation Time' value to pass through all three reference shapes, (with a choice of interpolation methods)